A rotorcraft, sometimes referred to a rotary wing aircraft, is fitted with a main rotor (possibly a plurality of main rotors) of substantially vertical axis and of large diameter that provide all or part of its lift.
In the helicopter configuration, the main rotor, which is driven by at least one engine, serves both to provide lift and propulsion. In principle, a helicopter also has an auxiliary rotor, referred to as a tail rotor or an “anti-torque” rotor for controlling the aircraft in yaw.
More precisely, the main rotor is constituted by some number of blades that may be considered as wings of large aspect ratio, and that are driven to rotate.
The blades are attached to a central portion of the rotor known as the “hub”.
At least one engine delivers mechanical power to the main and tail rotors, and also to auxiliary members.
The engine is generally a turbine engine. Given the number of revolutions at the outlet from a turbine is of the order of 30,000 revolutions per minute (rpm), which amounts to about 500 revolutions per second (rps), whereas the number of revolutions of the main rotor is of the order of 300 rpm, transmitting power from the turbine to the main rotor requires a speed-reducing unit to be interposed between them that is referred to as the main gearbox (MGB).
An outlet shaft from the MGB thus serves in particular to drive the main rotor and thus the blades via the hub.
Under these conditions, a rotorcraft carries out three kinds of flight, in principle:                vertical flight, upwards or downwards;        hovering flight, the aircraft remaining stationary relative to the air; and        flight in horizontal or sloping translation.        
In vertical flight, the resultant aerodynamic force and the weight comprise two forces acting in opposite directions on the same axis: flight is up or down depending on whether the aerodynamic force is greater than or less than the weight of the aircraft.
Hovering corresponds to the stationary aircraft being in equilibrium while subjected to two forces that are equal and opposite, i.e. the resultant aerodynamic force and the weight of the aircraft.
Flying in translation corresponds to a normal possibility for aircraft that are to move through the air: this feature is not specific to rotorcraft but it differs from the way airplanes fly.
In practice, the invention relates to a rotorcraft while in downward flight.
Under such circumstances, the flow of air generated by the main rotor differs depending on whether the aircraft is descending fast, moderately, or slowly.
Fast and moderate downward flight takes place under “non-powered” conditions: power is delivered by the stream of air and a freewheel is interposed in the power transmission assembly so as to enable the rotor to turn freely.
In contrast, slow downward flight takes place under powered conditions, the pilot causing the rotorcraft to descend under control by reducing the collective pitch of the blades of the main rotor.
The invention relates more specifically to slow downward flight of a rotorcraft, where such descent may take place vertically or else along a flight path having a steep slope, i.e. with the rotorcraft having a certain amount of forward horizontal speed referred to as instantaneous proper airspeed VP, this instantaneous proper speed remaining within a range of values that are relatively low, and being associated with an instantaneous vertical speed v.
During slow downward flight, and as explained below, a wake forms at the bottom portion of the main rotor, thereby constraining the bottom central streamlines of air to turn downwards and the top central streamlines of air to create a turbulent zone towards the periphery of the blades. The aerodynamic flow is thus disturbed and there is thus a risk of peripheral vortexes developing and completely isolating the plane of the rotor. This dangerous phenomenon, known as the “vortex state” leads to a general loss of lift and controllability.
In other words, when a rotorcraft begins to descend at a slow speed, the stream of air that, in translation flight, normally passes through the rotor in an upward direction runs the risk of being reversed and prevented from passing through the rotor, whether upwards or downwards: the blades then work in their own wash and the surrounding air forms a “vortex ring” in the vicinity of the main rotor.
A physical interpretation of the phenomenon is that on going downwards in this way the rotor “swallows up” its own wake, thereby explaining the turbulent nature of the flow observed under such circumstances.
Consequently, this mode of operation is characterized by the air above the rotor separating, giving rise to a wake.
The vortex ring generally develops when the helicopter is flying at a vertical speed close to the speed induced by the rotor, i.e. about 10 meters per second (m/s), in association with a low speed in translation: a large portion of the rotor is then in a stall zone, the various blade elements then working at an angle of incidence that is relatively high. While moving in translation at a speed that is moderate or high, the wash from the rotor is disposed of rearwards, such that the vortex state does not occur.
The vortex regime is dangerous, but the pilot can easily escape therefrom, either by beginning to move in translation (using the cyclic pitch control of the rotor blades), or by increasing the vertical speed (reducing the general pitch of the rotor blades) in order to cause the wash to disappear from the rotor.
For example, document U.S. Pat. No. 6,880,782 describes a device seeking to act on the rotor of a rotorcraft so as to escape from the vortex domain. Nevertheless, that type of device does not make it possible to prevent a rotorcraft from entering into the vortex domain. It does no more than correct a situation that is potentially dangerous, but it does not avoid such a situation occurring.
Furthermore, the document “Development of a helicopter vortex ring state warning system through a moving map display computer” by David Varnes relates to a rotorcraft approaching the vortex domain.